There are known transfer presses which include a transfer system for conveying workpieces to be pressed through a series of work stations in timed relation with performance of a series of pressing operations. Typically, the transfer system comprises a pair of feed bars juxtaposed so as to extend in a workpiece transferring direction and cross bars each spanned between these feed bars. The transfer system conveys workpieces held by vacuum caps by vacuum adsorption, these vacuum caps being attached to the respective cross bars. Alternatively, the transfer system conveys workpieces gripped at both sides by fingers attached to the feed bars. In this case, the pair of feed bars perform two-dimensional or three-dimensional movement to transfer an individual workpiece from one station where a set of dies are disposed to the next adjacent station where another set of dies are disposed.
The most typical method for driving the feed bars is a mechanical driving method in which the feed bars are driven, being linked to the press system with a cam and linkage mechanism. This method however reveals the disadvantage that adjustment at the time of die replacement is extremely difficult and therefore the individual driving method (such as disclosed in Japanese Patent Laid-Open Publication No. 6-218458 (1994)) becomes prevailing recently according to which the feed bars are driven with motors (servo motors) different from the motor for the press system. Transfer presses having a transfer system (servo transfer system) driven by the individual driving method has the advantage that the transfer system can be changed arbitrarily in its movement and has a simple configuration.
If the above transfer system encounters abnormal conditions such as when excessive load is imposed on a servo motor for one of the drive bars during operation, the drive bars would lose synchronization and be brought to an emergency stop. If such a malfunction occurs, all the drive bars need to be moved from their respective stop positions to a specified position in order to restore the drive bars to synchronization. In this case, if all the drive bars are moved to the specified position X at the same speed, the arrival times of the respective drive bars vary according to the distances of their stop positions from the specified position X, as understood from FIG. 6. In some cases, the movement of the drive bars to the specified position causes the rigid, feed bars Y to be distorted with an excessive force.
The present invention is directed to overcoming the above problem and one of the objects of the invention is therefore to provide a synchronization restoring system which is capable of safely restoring the drive bars of a servo transfer system to synchronization, while avoiding mechanical damage as much as possible.
It should be noted that the definition of the feed bars disclosed herein include lift beams such as described in Japanese Patent Laid-Open Publication No. 6-218458, the lift beams having cross bars that are mounted thereto so as to be freely movable in a workpiece transferring direction and being designed to ascend and descend but not to move in the transferring direction.